Interplay of electronic and lattice degrees of freedom in $A_{1-x}$Fe${}_{2-y}$Se${}_2$ superconductors under pressure

The local structure and electronic properties of Rb${}_{1-x}$Fe${}_{2-y}$Se${}_2$ are investigated by means of site selective polarized x-ray absorption spectroscopy at the iron and selenium $K$ edges as a function of pressure. A combination of dispersive geometry and a nanodiamond anvil pressure cell has permitted us to reveal a steplike decrease in the Fe-Se bond distance at $p\simeq 11$ GPa and, at the same pressure, the local atomic structure of Rb${}_{1-x}$Fe${}_{2-y}$Se${}_2$ becomes more ordered. Thus, at this pressure the phase separation might be suppressed, which has a large influence on the electronic structure, as evidenced by changes in the Fe $K$-edge prepeak. The present results provide compelling evidence that the reemerging superconductivity in $A_{1-x}$Fe${}_{2-y}$Se${}_2$ is closely related to the transition to a locally more ordered state, which has a different electronic structure.

Figure 1

(a) Normalized Fe $K$-edge XANES of Rb${}_{1-x}$Fe${}_{2-y}$Se${}_2$ at three pressures at room temperature. The pre-edge peak A and the two near-edge features B and C are indicated with dotted lines. The inset shows the pressure evolution of the prepeak area after subtraction of the background. (b) The first derivative of the Fe $K$-edge XANES for pressures ranging from $p\simeq 0.3$ to 15 GPa in $\simeq$1 GPa steps. The thicker lines are the corresponding ones shown in (a) with the same color code. (c) Pressure dependence of the inflection point ($\alpha$) representing the chemical potential. The dashed lines are a guide to the eyes.

Figure 2

(a) Normalized Se $K$-edge XANES of Rb${}_{1-x}$Fe${}_{2-y}$Se${}_2$ at room temperature for three pressures. (b) The first derivative of the Se $K$ edge for all pressures ranging from ambient pressure to 15 GPa in $\simeq$1 GPa steps. The thicker lines are the corresponding ones shown in (a) with the same color code. (c) Pressure dependence of the maximum of the multiple scattering peak M. The dashed line in (c) is a guide to the eyes.

Figure 3

The EXAFS oscillations of Rb${}_{1-x}$Fe${}_{2-y}$Se${}_2$ at room temperature extracted from the x-ray absorption spectra for selected pressures of (a) Se and (b) Fe $K$ edges. The data are shifted vertically for clarity of presentation. The measured data points are shown as symbols whereas the first shell modeling results involving the Fe-Se bond distance for the Se $K$ edge and the Fe-Se and Fe-Fe bond distances for the Fe $K$ edge are shown as solid black lines. Fourier transform (FT) magnitudes of the EXAFS oscillations corresponding to (c) Se $K$ edge and (d) Fe $K$ edge.

Figure 4

(a) Pressure dependence of the FT of the Fe $K$-edge EXAFS of Rb${}_{1-x}$Fe${}_{2-y}$Se${}_2$ at room temperature in a two-dimensional (2D) contour plot. (b) The Fe-Se distance (left axis) and the corresponding Debye-Waller factor ${\sigma }_{\mathrm{Fe}-\mathrm{Se}}^2$ (right axis) derived from the first shell analysis of the Se $K$-edge EXAFS. (c) The pressure dependence of the chemical potential with respect to ambient pressure (left axis) and the schematic pressure dependence of the superconducting transition temperature $T_{\mathrm{c}}$ adapted from Ref. 16 (right axis).